Our reason to use the Passenger Enterprise version is that it's the only truly reliable application server for Ruby that handles threads good enough. The administration tools and how it handles stuff like deployment upgrades to a huge application server are done remarkably better than in any other Ruby app server.
Update process loads our application once and forks it to all the workers like Unicorn does. Puma wanted to load the application as many times as we had workers, which caused a huge spike every time we deployed.
Same goes to the thread handling. It really seems Passenger is the only Ruby server able to handle massive loads with threaded Rails app. Money well spent.